Are you ready for winter? There’s a chance of snow in the mountains of Wyoming today, and before you know it, everyone everywhere will start grumbling about the cold. The big question on everyone’s mind is what’s in store for us this winter, and El Niño will likely be a major player in the coming months. Most indications point to the chance that the abnormally warm water in the Pacific will have a significant effect on our weather here in the United States.

What Is El Niño?

We talk about El Niño so often that we forget that there are people who don’t know what it is, leading to the widespread misconception that the phenomenon is a destructive storm or weather pattern in its own right, that it’s something that can “hit” California, as so many news stories have falsely reported in recent months.

An El Niño is the abnormal warming of waters in the eastern Pacific Ocean around the equator. The warming is measured using regions—the most commonly used region, “Niño 3.4,” is pictured above—and the sea surface temperature (SST) anomalies in these regions are used to determine whether or not we have an El Niño (sustained above-average SSTs) or a La Niña (sustained below-average SSTs).

The exact definition of an El Niño varies slightly from source to source; the Climate Prediction Center says that sea surface temperature anomalies have to be +0.5°C or greater for five consecutive three-month periods (in other words, seven consecutive months) in order to classify the warming event an honest-to-God(zilla?) El Niño.

An El Niño occurs when trade winds in the equatorial Pacific slow down or even reverse direction, allowing warmer water to pool-up on the eastern side of the ocean where it doesn’t belong. Scientists are still trying to figure out exactly what triggers this process, as well as ways of more accurately predicting when this shift will occur.

How Strong Will It Get?

In its weekly update on September 14, the Climate Prediction Center’s analysis showed that average sea surface temperature anomalies in that Niño 3.4 region are +2.3°C, which is mighty toasty.

If you have a body temperature considered to be average (98.6°F), that kind of anomaly would give you a fever of 102.7°F. It’s not the same, of course, but it helps explain the way relatively small changes in temperature can have a big impact on the environment. That kind of fever could knock you down and make you see purple unicorns dancing on the neighbor’s roof, just as that kind of anomaly in sea surface temperatures can dramatically alter the atmosphere, affecting weather patterns around the world.

Most models and forecasters expect that the waters will continue warming through the winter, with the current El Niño peaking around the end of the year before slowly subsiding as we head into 2016.

We’ve seen lots of chatter about the fact that this event could be the strongest El Niño ever recorded. If these regions continue warming as models project, this could very well be the strongest one on record—exceeding the current record held by the El Niño of 1997-1998—but that’s a tricky subject because the sample size is so small. If we had hundreds of years of sea surface temperature data instead of just six-and-a-half decades of reliable information, declaring this “the strongest El Niño ever recorded” would have a little more oomph behind it. It’s still noteworthy, though, and it could already be having serious consequences on our weather.

Why It Alters Our Weather

In the Northern Hemisphere, our weather is largely affected by two jet streams—the polar jet stream and the sub-tropical jet stream. The pattern we typically see in the winter is dominated by the polar jet stream—usually centered over Canada and the northern United States—allowing strong low pressure systems and brutally cold Arctic air to sweep down and freeze us all into misery.

During an El Niño, the warmer sea surface temperatures alter our weather by nudging both the polar and sub-tropical jets north into the higher latitudes. This readjustment keeps colder, snowier weather farther north and allows the sub-tropical jet stream to generate storms that sweep across the southern half of the U.S. from west to east.

Hurricanes

While it doesn’t really deal with winter, we’re at the beginning of fall and hurricanes are the most immediate oceanic threat to the United States at the moment, so allow me to mention them for a second.

One of the well-advertised summer effects of El Niño is the potential for an above-average number of tropical cyclones in the eastern Pacific. This part of the world typically sees 15 named storms, and so far this year—with about two months left in the season—that number is up to 12.

What’s even more astounding is that 9 of those 12 named storms made it to hurricane status in the eastern Pacific (before they crossed 140°W into the central Pacific), and 6 (SIX!) of those hurricanes went on to become category four hurricanes before they dissipated. The water in this part of the ocean is very warm, and it’s warm enough to sustain a few more strong storms before the season is out.

What does this mean for the United States? The remnants of tropical cyclones that move close to the Baja Peninsula can inject atmospheric moisture into the southwestern part of the country, bringing the potential for heavy rain from southern California through Colorado.

We saw that just this weekend when the remnants of Hurricane Linda spat moisture northward and allowed showers and thunderstorms to develop in southern California. Parts of Los Angeles saw more than two inches of rain in one day, coming in as the wettest day of the year across the region.

It’s not a guarantee that we’ll see this happen again before tropical cyclone season shuts down, but with abnormally warm waters in this part of the Pacific Ocean, it’s possible we could see a few more cyclones before the year is out, and any one of them, should they track reasonably close to the Baja, could bring moisture to the southwest.

Precipitation Trends

Back to winter now. One of the classic effects of an El Niño in the United States is the uptick in precipitation in the southern half of the country. The Climate Prediction Center released a new batch of seasonal outlooks this morning, and their thinking for the December-January-February time frame heavily resembles the precipitation and temperature trends you’d see in a meteorology textbook’s chapter on El Niño.

According to their latest outlook, the agency is increasingly confident in the odds of above-average precipitation from California to the Mid-Atlantic, with the greatest odds near the borders and coasts. However, if you live in a place like California, don’t bank on the potential rain this winter to eliminate your devastating drought.

Many spots in the northern parts of the United States could see below-average precipitation, which is to be expected during an El Niño. This is terrible news for drought-stricken areas of the Rockies, where persistent dry conditions will exacerbate water troubles and the wildfire situation.

Temperature Trends

Temperature anomalies in the United States also somewhat follow precipitation trends, with cooler-than-average weather possible in the south and warmer-than-average temperatures possible along the West Coast and up along the Canadian border.

A Word of Caution

This doesn’t mean that these precipitation and temperature trends will happen, just that they could happen based on current patterns and what we know about how the atmosphere responds to El Niños. The south could see a warm, dry spell and the north could still see a few sub-zero days with blizzards.

We’re talking about the odds of certain trends, here. It’s an outlook, a general idea of what you might expect given the evidence as it stands today.

Snow? SNOW!? SNOOOOW!!!

I can see it now: the comments will be full of questions about snow. I’m not touching this one with a ten-foot shovel. The Washington Post’s Capital Weather Gang wrote an article on snow prospects in the capital city during strong El Niño years, and they found the city either saw beaucoup snow or nothing at all. Feast or famine. Not exactly what you want to hear, no matter which side of the snow issue you fall on.

It’s hard to pinpoint snow totals days in advance, let alone months.

Prepare for snow either way. It’s notoriously hard to see snow in the south, so the farther south you, the more you should prepare for freezing rain and sleet instead of snow. It just takes one snow or ice storm to make a humdrum winter the worst anyone in the region can remember. The chance of wintry precipitation depends on the exact tracks of storms—ask New York City how that goes.

[Images: NOAA, author, NOAA, NWS, CPC]

Email: dennis.mersereau@gawker.com | Twitter: @wxdam

If you enjoy The Vane, then you’ll love my upcoming book, The Extreme Weather Survival Manual, which comes out on October 6 and is now available for pre-order on Amazon.